A Brief Sketch of Kant’s Critical Philosophy

Metaphysical knowledge can be either dogmatic or critical. Dogmatic metaphysics seeks to know things as they are in themselves. Critical metaphysics, which Kant calls “Critique”,
only gives us knowledge of things as they must appear to us, and hence
of the necessary features of all possible experience. Dogmatic
metaphysics would have to meet two requirements which are inconsistent
in Kant’s system. First, it would have to be synthetic a priori. It
could not be analytic a priori, for then it could not give us new
knowledge. Neither could it be synthetic a posteriori, for then it could
tell us no more than natural science does. Second, it would have to go
beyond the bounds of all possible experience; otherwise, it would not be
distinct from mathematics and geometry, which, while also synthetic a
priori, are limited to possible experience. This limitation is what
makes them possible, for as we said above, they are “built into” space
and time as forms of our sensibility. Anything which can appear to us
must be subject to our forms of sensibility, and so mathematics and
geometry must hold of all appearances. But since dogmatic metaphysics is
supposed to apply to things which cannot appear to us, we cannot know a
priori what they are like, for they are not subject to the only
conditions under which experience, and hence synthetic a priori
knowledge, is possible. In consequence, metaphysical knowledge of a
dogmatic sort is impossible. Now we can see the source of Kant’s
distaste for dogmatic metaphysics: It poses questions which it cannot
answer.